Gear module converter
Convert gear tooth size between module and diametral pitch. Two gears mesh only if this number matches.
That's 1.02× as coarse as a car gearbox, while a truck final drive is 2.4× coarser. A 20-tooth gear of this size would be 50.8 mm across.
What it means
How big are those teeth?
Module and pitch describe tooth size only. Gears also need the same pressure angle, usually 20°, before they will run together.
Every unit
Your tooth size in every unit
Learn
Three things worth knowing
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module = 25.4 ÷ DP
So 10 DP is module 2.54, which does not equal module 2.5. Near-misses like this are why metric and imperial gears grind themselves apart.
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Counting teeth per inch
Imperial practice fixed the number of teeth per inch of diameter, so gears in one family always meshed. Metric practice inverted it into a length, which is easier to measure with calipers.
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Bigger DP is smaller teeth
Module grows with tooth size; diametral pitch shrinks. A 24 DP gear is fine-toothed, a 4 DP gear is coarse.
Shortcuts
Popular tooth size conversions
Background
About module and diametral pitch
Module is millimeters of pitch diameter per tooth: a module 2 gear with 20 teeth has a 40 mm pitch circle. Diametral pitch is the imperial opposite — teeth per inch of diameter — so a bigger DP means smaller teeth.
They are reciprocals through the inch: module = 25.4 ÷ DP.